Nerve and Muscle Physiology Practice Questions

Q: Magnitude of action potential is mainly affected by:

Sodium ion. ### Explanation **1. Why Sodium Ion is Correct:** The **magnitude (amplitude)** of an action potential is primarily determined by the concentration gradient of sodium ions ($Na^+$) across the cell membrane. According to the **Hodgkin-Huxley model**, the peak of the action potential approaches the **Equilibrium Potential of Sodium ($E_{Na}$)**, which is approximately $+60\ mV$. * During depolarization, voltage-gated $Na^+$ channels open, allowing a massive influx of $Na^+$. * If the extracellular $Na^+$ concentration decreases (hyponatremia), the concentration gradient weakens, leading to a lower peak and reduced magnitude of the action potential. **2. Why Other Options are Incorrect:** * **Potassium Ion ($K^+$):** $K^+$ primarily determines the **Resting Membrane Potential (RMP)** and the **repolarization** phase. Changes in $K^+$ affect the excitability of the cell (threshold) rather than the peak magnitude of the spike. * **Calcium Ion ($Ca^{2+}$):** Extracellular $Ca^{2+}$ acts as a "gatekeeper" for $Na^+$ channels. Low $Ca^{2+}$ (hypocalcemia) lowers the threshold for firing (causing tetany), but it does not significantly dictate the peak magnitude of the action potential in nerve/skeletal muscle. * **Hydrogen Ion ($H^+$):** pH changes affect overall protein function and excitability (acidosis depresses excitability; alkalosis increases it), but $H^+$ is not a primary charge carrier for the action potential spike. **3. Clinical Pearls for NEET-PG:** * **RMP** is mainly dependent on **Potassium** (due to high resting permeability). * **Action Potential Magnitude** is mainly dependent on **Sodium**. * **Hypocalcemia** increases neuronal excitability (Tetany) by lowering the threshold potential. * **Hyperkalemia** initially increases excitability (brings RMP closer to threshold) but eventually causes inactivation of $Na^+$ channels, leading to paralysis or cardiac arrest.

Q: Fast axonal transport is a component of orthograde axonal transport. What is the rate of fast axonal transport?

400 mm/d. **Explanation:** Axonal transport is the process by which organelles, proteins, and vesicles are moved along the axon. It is categorized based on direction (Orthograde/Anterograde vs. Retrograde) and speed (Fast vs. Slow). **1. Why Option C is Correct:** **Fast Axonal Transport** (specifically the orthograde component) occurs at a rate of approximately **400 mm/day**. This process is mediated by the motor protein **Kinesin**, which "walks" along microtubules using ATP. It is responsible for transporting membrane-bound organelles, mitochondria, and neurotransmitter vesicles from the cell body to the axon terminals. **2. Analysis of Incorrect Options:** * **Options A and B (100–200 mm/d):** These rates are too slow for fast transport. While some sources cite a range (200–400 mm/d), **400 mm/day** is the standard value cited in major physiology textbooks (like Guyton and Ganong) and is the high-yield figure for NEET-PG. * **Option D (600 mm/d):** This exceeds the physiological rate of standard fast axonal transport in human neurons. **3. Clinical Pearls & High-Yield Facts:** * **Slow Axonal Transport:** Occurs at a rate of **0.5 to 10 mm/day**. It transports structural elements like the axoplasmic matrix and cytoskeletal proteins (neurofilaments). * **Retrograde Transport:** Moves from the periphery to the cell body at a rate of **~200 mm/day** via the motor protein **Dynein**. * **Clinical Relevance:** Certain neurotropic viruses (e.g., **Rabies, Herpes Simplex**) and toxins (e.g., **Tetanus toxin**) exploit **Retrograde transport** to reach the Central Nervous System. * **Mnemonic:** **K**inesin moves to the **K**ick-off (Anterograde/Forward); **D**ynein moves to the **D**en (Retrograde/Backward).

Q: Among the following nerve fibers, which one has the lowest conduction velocity?

C. **Explanation:** The conduction velocity of a nerve fiber is primarily determined by two factors: **myelination** and **fiber diameter**. According to the Erlanger-Gasser classification, nerve fibers are categorized into Types A, B, and C based on these characteristics. **Why Option D is Correct:** **Type C fibers** are the only **unmyelinated** fibers in the human body. They have the smallest diameter (0.4–1.2 μm) and the slowest conduction velocity (0.5–2.0 m/s). Because they lack the insulating myelin sheath, they cannot perform saltatory conduction and instead rely on slow, continuous propagation of action potentials. They primarily transmit "slow" pain, temperature, and post-ganglionic autonomic signals. **Why Other Options are Incorrect:** * **A alpha (Aα):** These are the thickest, most heavily myelinated fibers (12–20 μm) with the fastest conduction velocity (70–120 m/s). They handle proprioception and somatic motor functions. * **A beta (Aβ):** These are large, myelinated fibers involved in touch and pressure. Their velocity (30–70 m/s) is significantly higher than Type C. * **A gamma (Aγ):** These myelinated fibers supply muscle spindles (intrafusal fibers). While slower than Aα and Aβ, their velocity (15–30 m/s) is still much faster than the unmyelinated C fibers. **High-Yield Clinical Pearls for NEET-PG:** * **Order of Susceptibility:** * **Local Anesthetics:** Block **Type C** fibers first (smallest diameter), which is why pain is the first sensation lost. * **Hypoxia:** Affects **Type A** fibers first (most metabolically active). * **Pressure:** Affects **Type A** fibers first (e.g., "limb falling asleep"). * **Conduction Velocity Formula:** For myelinated fibers (Type A), Velocity (m/s) ≈ 6 × Diameter (μm). * **Pain Transmission:** **Aδ fibers** carry "fast/sharp" pain, while **C fibers** carry "slow/dull/aching" pain.

Q: What does the 'all or none' phenomenon mean in physiology?

An action potential has a fixed, all-or-nothing value.. ### Explanation The **All-or-None Law** is a fundamental principle of neurophysiology. It states that if a stimulus is strong enough to reach the **threshold potential** (typically -55mV), an action potential will be generated at a constant, maximum amplitude. If the stimulus is subthreshold, no action potential occurs at all. **Why Option B is Correct:** Once the threshold is reached, voltage-gated sodium channels open in a regenerative cycle. The resulting action potential is independent of the stimulus strength; its amplitude and shape remain **fixed and constant** for that specific nerve or muscle fiber. Increasing the intensity of the stimulus will not increase the size of the action potential. **Analysis of Incorrect Options:** * **Option A & D:** These describe **graded potentials** (like receptor potentials or EPSPs), where the magnitude varies with the strength or duration of the stimulus. In contrast, action potentials are non-graded. * **Option C:** Subthreshold stimuli only cause local electronic changes (local response) but fail to trigger the rapid depolarization required for an action potential. **High-Yield NEET-PG Pearls:** * **Frequency Coding:** Since the amplitude of an action potential is fixed, the intensity of a stimulus is communicated to the CNS by the **frequency** of action potentials, not their size. * **Applicability:** The All-or-None law applies to **individual** nerve fibers and motor units. It does **not** apply to a whole nerve trunk (which contains many fibers with different thresholds) or a whole skeletal muscle, which show graded responses via recruitment. * **Refractory Period:** This ensures that action potentials remain discrete events, preventing them from merging or summing together.

Q: Rheobase indicates:

Magnitude of current. ### Explanation **Rheobase** is defined as the **minimum intensity (magnitude) of a constant electrical current** that, when applied for an indefinite period, is required to produce an action potential in an excitable tissue (nerve or muscle). #### Why Option A is Correct: The Strength-Duration curve illustrates the relationship between the intensity of a stimulus and the time required to excite a tissue. Rheobase represents the **threshold intensity** on the Y-axis. If the current magnitude is below the rheobase, the tissue will not fire, regardless of how long the stimulus is applied. #### Why Other Options are Incorrect: * **B. Rate of discharge:** This refers to the frequency of action potentials, which is determined by the intensity of a suprathreshold stimulus and the refractory period, not the rheobase. * **C. Velocity of nerve conduction:** This is a measure of speed (distance/time) determined by myelination and axon diameter, unrelated to the initial threshold intensity. * **D. Specificity of impulse transmission:** This refers to the "all-or-none" law or synaptic pathways, rather than the electrical parameters of stimulation. --- ### High-Yield Facts for NEET-PG: * **Chronaxie:** The minimum **time** required for a current of **double the rheobase** strength to excite the tissue. * **Excitability Inverse Relationship:** Chronaxie is inversely proportional to excitability. A shorter chronaxie means the tissue is more excitable. * **Chronaxie Values:** Nerve (0.1 ms) < Skeletal Muscle (0.25 ms) < Cardiac Muscle (2.0 ms) < Smooth Muscle (high). * **Utilization Time:** The minimum time required to excite a tissue using a current exactly at the rheobase strength. (Note: This is less clinically useful than chronaxie because it is difficult to measure accurately).

Q: Repolarization of a nerve is due to:

Potassium ions. **Explanation:** The action potential of a nerve fiber consists of two primary phases: depolarization and repolarization. **Repolarization** is the process by which the membrane potential returns to its negative resting state after depolarization. This is primarily achieved by the **efflux (outward movement) of Potassium ions ($K^+$)**. During this phase, voltage-gated $Na^+$ channels close (inactivate), and voltage-gated $K^+$ channels open. Since the concentration of $K^+$ is higher inside the cell, these ions rush out, removing positive charges from the interior and restoring the electronegativity of the cell. **Analysis of Incorrect Options:** * **Sodium ions ($Na^+$):** These are responsible for **Depolarization**. The rapid influx of $Na^+$ into the cell makes the interior positive. * **Calcium ions ($Ca^{2+}$):** While $Ca^{2+}$ is crucial for neurotransmitter release at the synapse and the "plateau phase" in cardiac muscle action potentials, it is not the primary ion responsible for repolarization in nerve fibers. * **Hydrogen ions ($H^+$):** These ions influence the pH of the environment but do not play a direct role in the ionic flux of a standard nerve action potential. **High-Yield NEET-PG Pearls:** * **Resting Membrane Potential (RMP):** Primarily maintained by $K^+$ "leak" channels (RMP of a typical neuron is -70 mV). * **Hyperpolarization:** Occurs when $K^+$ channels remain open slightly longer than necessary, moving the potential closer to the $K^+$ equilibrium potential (-94 mV). * **Na+-K+ ATPase Pump:** Does not cause repolarization but restores the ionic gradients (3 $Na^+$ out, 2 $K^+$ in) *after* the action potential cycles are complete.

Question 1

Magnitude of action potential is mainly affected by:

Accepted Answer

Sodium ion

Answer

Calcium ion

Answer

Hydrogen ion

Answer

Potassium ion

Question 2

Fast axonal transport is a component of orthograde axonal transport. What is the rate of fast axonal transport?

Accepted Answer

400 mm/d

Answer

100 mm/d

Answer

200 mm/d

Answer

600 mm/d

Question 3

Among the following nerve fibers, which one has the lowest conduction velocity?

Accepted Answer

C

Answer

A alpha

Answer

A beta

Answer

A gamma

Question 4

At the neuromuscular junction:

Accepted Answer

The motor nerve endings secrete acetylcholine.

Answer

The muscle membrane possesses muscarinic receptors.

Answer

The motor nerve endings secrete norepinephrine.

Answer

Curare leads to prolongation of neuromuscular transmission.

Question 5

What does the 'all or none' phenomenon mean in physiology?

Accepted Answer

An action potential has a fixed, all-or-nothing value.

Answer

The magnitude of a response is related to the strength of the stimulus.

Answer

Action potentials can be developed by subthreshold stimuli.

Answer

The magnitude of a response is related to the duration of the stimulus.

Question 6

Rheobase indicates:

Accepted Answer

Magnitude of current

Answer

Rate of discharge

Answer

Velocity of nerve conduction

Answer

Specificity of impulse transmission

Question 7

Which of the following has the least conduction velocity?

Accepted Answer

Somatic sensory neuron

Answer

Somatic motor neuron

Answer

Autonomic sensory neuron

Answer

Autonomic motor neuron

Question 8

The 'all or none' law is obeyed by which of the following potentials?

Accepted Answer

Action potential

Answer

Postsynaptic potential

Answer

Non-propagated potential

Answer

Spike potential

Question 9

All of the following statements regarding activity at the neuromuscular junction are true EXCEPT:

Accepted Answer

Ach stimulates muscarinic cholinergic receptors on the muscle endplate.

Answer

Action potentials on the alpha motor neuron release acetylcholine (Ach) from the axon terminal.

Answer

Cholinergic receptors on the muscle endplate are cation channels.

Answer

Alpha motor neurons excite, and Renshaw cells inhibit, the muscle endplate.

Question 10

Repolarization of a nerve is due to:

Accepted Answer

Potassium ions

Answer

Hydrogen ions

Answer

Sodium ions

Answer

Calcium ions

Nerve and Muscle Physiology — MCQs

Nerve and Muscle Physiology — MCQs

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